Re: [PATCH v7] seccomp, ptrace: add support for dumping seccomp filters

From: Kees Cook
Date: Tue Oct 13 2015 - 13:10:11 EST

On Tue, Oct 13, 2015 at 7:47 AM, Tycho Andersen
<tycho.andersen@xxxxxxxxxxxxx> wrote:
> This patch adds support for dumping a process' (classic BPF) seccomp
> filters via ptrace.
>
> PTRACE_SECCOMP_GET_FILTER allows the tracer to dump the user's classic BPF
> seccomp filters. addr should be an integer which represents the ith seccomp
> filter (0 is the most recently installed filter). data should be a struct
> sock_filter * with enough room for the ith filter, or NULL, in which case
> the filter is not saved. The return value for this command is the number of
> BPF instructions the program represents, or negative in the case of errors.
> Command specific errors are ENOENT: which indicates that there is no ith
> filter in this seccomp tree, and EMEDIUMTYPE, which indicates that the ith
> filter was not installed as a classic BPF filter.
>
> A caveat with this approach is that there is no way to get explicitly at
> the heirarchy of seccomp filters, and users need to memcmp() filters to
> decide which are inherited. This means that a task which installs two of
> the same filter can potentially confuse users of this interface.
>
> v2: * make save_orig const
> * check that the orig_prog exists (not necessary right now, but when
> grows eBPF support it will be)
> * s/n/filter_off and make it an unsigned long to match ptrace
> * count "down" the tree instead of "up" when passing a filter offset
>
> Signed-off-by: Tycho Andersen <tycho.andersen@xxxxxxxxxxxxx>
> CC: Kees Cook <keescook@xxxxxxxxxxxx>

Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>

This looks good to me. I'd love to hear an "ok" from ptrace folks, but
I think this is ready.

-Kees

> CC: Will Drewry <wad@xxxxxxxxxxxx>
> CC: Oleg Nesterov <oleg@xxxxxxxxxx>
> CC: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> CC: Pavel Emelyanov <xemul@xxxxxxxxxxxxx>
> CC: Serge E. Hallyn <serge.hallyn@xxxxxxxxxx>
> CC: Alexei Starovoitov <ast@xxxxxxxxxx>
> CC: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
> ---
> include/linux/seccomp.h | 11 +++++++
> include/uapi/linux/ptrace.h | 2 ++
> kernel/ptrace.c | 5 +++
> kernel/seccomp.c | 77 ++++++++++++++++++++++++++++++++++++++++++++-
> 4 files changed, 94 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h
> index f426503..2296e6b 100644
> --- a/include/linux/seccomp.h
> +++ b/include/linux/seccomp.h
> @@ -95,4 +95,15 @@ static inline void get_seccomp_filter(struct task_struct *tsk)
> return;
> }
> #endif /* CONFIG_SECCOMP_FILTER */
> +
> +#if defined(CONFIG_SECCOMP_FILTER) && defined(CONFIG_CHECKPOINT_RESTORE)
> +extern long seccomp_get_filter(struct task_struct *task,
> + unsigned long filter_off, void __user *data);
> +#else
> +static inline long seccomp_get_filter(struct task_struct *task,
> + unsigned long n, void __user *data)
> +{
> + return -EINVAL;
> +}
> +#endif /* CONFIG_SECCOMP_FILTER && CONFIG_CHECKPOINT_RESTORE */
> #endif /* _LINUX_SECCOMP_H */
> diff --git a/include/uapi/linux/ptrace.h b/include/uapi/linux/ptrace.h
> index a7a6979..c9d0b21 100644
> --- a/include/uapi/linux/ptrace.h
> +++ b/include/uapi/linux/ptrace.h
> @@ -23,6 +23,8 @@
>
> #define PTRACE_SYSCALL 24
>
> +#define PTRACE_SECCOMP_GET_FILTER 40
> +
> /* 0x4200-0x4300 are reserved for architecture-independent additions. */
> #define PTRACE_SETOPTIONS 0x4200
> #define PTRACE_GETEVENTMSG 0x4201
> diff --git a/kernel/ptrace.c b/kernel/ptrace.c
> index 787320d..b760bae 100644
> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -1016,6 +1016,11 @@ int ptrace_request(struct task_struct *child, long request,
> break;
> }
> #endif
> +
> + case PTRACE_SECCOMP_GET_FILTER:
> + ret = seccomp_get_filter(child, addr, datavp);
> + break;
> +
> default:
> break;
> }
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index 06858a7..7bc8305 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -347,6 +347,7 @@ static struct seccomp_filter *seccomp_prepare_filter(struct sock_fprog *fprog)
> {
> struct seccomp_filter *sfilter;
> int ret;
> + const bool save_orig = config_enabled(CONFIG_CHECKPOINT_RESTORE);
>
> if (fprog->len == 0 || fprog->len > BPF_MAXINSNS)
> return ERR_PTR(-EINVAL);
> @@ -370,7 +371,7 @@ static struct seccomp_filter *seccomp_prepare_filter(struct sock_fprog *fprog)
> return ERR_PTR(-ENOMEM);
>
> ret = bpf_prog_create_from_user(&sfilter->prog, fprog,
> - seccomp_check_filter, false);
> + seccomp_check_filter, save_orig);
> if (ret < 0) {
> kfree(sfilter);
> return ERR_PTR(ret);
> @@ -867,3 +868,77 @@ long prctl_set_seccomp(unsigned long seccomp_mode, char __user *filter)
> /* prctl interface doesn't have flags, so they are always zero. */
> return do_seccomp(op, 0, uargs);
> }
> +
> +#if defined(CONFIG_SECCOMP_FILTER) && defined(CONFIG_CHECKPOINT_RESTORE)
> +long seccomp_get_filter(struct task_struct *task, unsigned long filter_off,
> + void __user *data)
> +{
> + struct seccomp_filter *filter;
> + struct sock_fprog_kern *fprog;
> + long ret;
> + unsigned long count = 0;
> +
> + spin_lock_irq(&current->sighand->siglock);
> + if (!capable(CAP_SYS_ADMIN) ||
> + current->seccomp.mode != SECCOMP_MODE_DISABLED) {
> + ret = -EACCES;
> + goto out_self;
> + }
> +
> + spin_lock_irq(&task->sighand->siglock);
> + if (task->seccomp.mode != SECCOMP_MODE_FILTER) {
> + ret = -EINVAL;
> + goto out_task;
> + }
> +
> + filter = task->seccomp.filter;
> + while (filter) {
> + filter = filter->prev;
> + count++;
> + }
> +
> + if (filter_off >= count) {
> + ret = -ENOENT;
> + goto out_task;
> + }
> + count -= filter_off;
> +
> + filter = task->seccomp.filter;
> + while (filter && count > 1) {
> + filter = filter->prev;
> + count--;
> + }
> +
> + if (WARN_ON(count != 1)) {
> + /* The filter tree shouldn't shrink while we're using it. */
> + ret = -ENOENT;
> + goto out_task;
> + }
> +
> + fprog = filter->prog->orig_prog;
> + if (!fprog) {
> + /* This must be a new non-cBPF filter, since we save every
> + * every cBPF filter's orig_prog above when
> + * CONFIG_CHECKPOINT_RESTORE is enabled.
> + */
> + ret = -EMEDIUMTYPE;
> + goto out_task;
> + }
> +
> + ret = fprog->len;
> + if (!data)
> + goto out_task;
> +
> + if (copy_to_user(data, fprog->filter, bpf_classic_proglen(fprog))) {
> + ret = -EFAULT;
> + goto out_task;
> + }
> +
> +out_task:
> + spin_unlock_irq(&task->sighand->siglock);
> +
> +out_self:
> + spin_unlock_irq(&current->sighand->siglock);
> + return ret;
> +}
> +#endif
> --
> 2.5.0
>



--
Kees Cook
Chrome OS Security
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